The general method of operation of such rotary valves is well known in the art of power steering design and so will not be described in any greater detail in this specification. A description of this operation is contained in U.S. Pat. No. 3,022,772 (Zeigler), commonly held as being the "original" patent disclosing the rotary valve concept. According to that patent, the input-shaft and sleeve are biased towards the centred position by a torsion bar spring. Thus when small input torques are applied to the steering wheel and hence the input-shaft, only a small magnitude of relative rotation occurs between the input-shaft and sleeve and, for such low valve operating angles, little power assistance is provided by the valve. For larger input torques proportionately greater valve operating angles are generated, leading to much greater levels of power assistance. The relationship between the level of power assistance generated in the valve, as a function of input torque, is known as the valve pressure characteristic. This valve pressure characteristic is therefore determined by the geometry of the changing orifice area as a function of valve operating angle.
Such rotary valves are nowadays regularly incorporated in firewall-mounted rack and pinion steering gears and, in this situation, any noises such as hiss emanating from the valve are very apparent to the driver. Hiss results from cavitation of the hydraulic oil as it flows in the orifices defined by the input-shaft metering edge contours and the adjacent edges of the sleeve slots particularly during times of high pressure operation of the valve such as during vehicle parking manoeuvres, where pressures typically as high as 8 MPa can be generated. It is well known in the art of power steering valves than an orifice is less prone to cavitation if the metering edge contour has a high aspect ratio of width to depth, thereby constraining the oil to flow as a thin sheet of constant depth all along any one metering edge contour, and that the flow of oil is divided equally amongst the aforementioned network of orifices, so further effectively increasing the aspect ratio. It is also well known that cavitation is less likely to occur if the metering edge contour, where it intersects the outside diameter of the input-shaft, is nearly tangential thereto, hence constituting a shallow chamfer of no more than about 1 in 12 slope. Typically, during high pressure operation of the rotary valve, one edge of each sleeve slot is angularly displaced about one half of a degree from the point of intersection of this chamfer with the input-shaft outside diameter, and the radial depth of the orifice so formed is about 0.012 mm.
As the input-shaft and sleeve are angularly displaced towards the centred position, cavitation is less likely to occur and additional chamfers of steeper slope are sometimes used further down the metering edge contour in order to generate the required pressure characteristic in the zone of valve operation associated with vehicle cornering.
Several manufacturers achieve the desired accuracy in the metering edge contour by grinding these chamfers in special machines in which the input-shaft is supported on centres previously used for cylindrically finish grinding its outside diameter. Such chamfer grinding machines incorporate a large diameter cylindrical grinding wheel, of a width equal to the axial extent of the metering edge contours, which is successively traversed across the edge of each input-shaft groove at varying radial heights with respect to the input-shaft axis.
Other manufacturers adapt, for this purpose, cam grinding machines similar to those used for example in the manufacture of camshafts for automobile engines, thread cutting taps, and router cutters, wherein the workpiece is supported on centres and rotated continuously while being cyclically moved towards and away from a grinding wheel under the action of a master cam. The required amount of stock is progressively removed by infeeding of the grinding wheel during many revolutions of the workpiece. As in the case of chamfer grinding machines, a large diameter grinding wheel is used, which makes it impossible to grind that part of the metering edge contour towards the centreline of the groove where increasing depth would cause the grinding wheel to interfere with the opposite edge of the same groove. This steeply sloping and relatively deep portion of the input-shaft metering edge contour will henceforth be referred to as the "inner" metering edge contour and its geometry generally affects the on-centre region of the valve pressure characteristic. This portion is generally manufactured by means other than the chamfer or cam grinding machines just described which, for reasons stated, are only capable of grinding the "outer" metering edge contour. This previously described gently sloping wedge shaped portion of the metering edge contour determines the valve pressure characteristic at medium and high operating pressures, as well as determining the valve noise characteristic.
The total valve operating angle from the centred position to the region of maximum operating pressure associated with vehicle parking is typically about 31/2 degrees, of which the inner and outer metering edge contours each control about one half. The junction between the inner and outer metering edge contours usually occurs in the intermediate range of valve operation associated with vehicle cornering, where it is important to have a progressive or approximately linear valve pressure characteristic without any discontinuities, thereby maximising the driver's control of the vehicle in this important mode of operation. To achieve this linear relationship, the outer metering edge contour, starting from the flat chamfer needed for hiss suppression, is required to have a spiral geometry of increasing curvature, which can provide the necessary relationship between orifice area and valve operating angle. For the practical manufacturing reasons described above, it is not possible to continue this spiral geometry on the inner portion of the metering edge contour, however the same type of orifice area versus valve operating angle relationship can be achieved by using metering edge contours of varying cross-sectional shape along their length, which can be manufactured by means other than grinding. This is acceptable as there is no longer the need for achieving uniform thin sheet flow as in the case of the outer metering edge contours due to the lower operating pressures associated with the inner metering edge contours. For example the required three dimensional metering edge geometries can be approximated by using milling or hobbing processes to form the input-shaft grooves. A better solution, without compromise, is available using the roll-imprinting process described in U.S. Pat. No. 4,651,551 (Bishop) which allows limitless three dimensional featuring on the sides of the grooves.